UPDATE:
low speed with any modem DNA A211-I adsl modem +wifi (2) Netgear DG 834G: 3 in one NEW
1.
The critical "blood test" shows your Download SNR is very low 5.8 dB and Blood pressure , I mean Attenuation 46 dB too high ! picture thanks to @meetdilip
http://img502.imageshack.us/ifs/246/.../2/snrattm.jpg
2.
I was misled by your earlier report that you got good speed when bsnl used their test modem/router !
3.
It does not make much difference whether you are in "Dialer" mode or "Always On" PPP/PPPoE mode. latter required to connect multiple computers, either in lan mode or wifi mode.
4.
The speed also will reduce when tested in wifi mode. Data transfer 54 Mbps compared to Lan 100 Mbps.
5.
The bandwidth is also shared with different computer/s.
Para two :
6.
Now you can do the same set up with DG834gmodem/router and compare only SNR/ATTN data. Mod mode GDMT usual .
7.
Reduce the number of phones ( max 3 or 4 ) Connected AFTER the ADSL splitter.
( No extension phone to be connected in any room without additional adsl splitters )
8.
Check contacts fro rusting/black joints/oxidization etc.
Use new Rj11 rj45 short cables. Inspect incoming BSNL line for high resistance
joints etc.
9.
If you are far away from the Exchange not much improvement expected.
10. Feedback ipconfig /all needed. in any mode. Lan and or wifi mode.
11. Since you have spare modem+wifi ,
can try FW upgrade for DNA A211-I . after trying other options.
(see link below. wa 3002-g4.)